This invention relates to a laser linear distance measurement system and relates more particularly to a retroreflector for such systems which obtains a precise direct accurate dimensional measurement while adapting to angular displacement or pitch which in prior systems effects an Abbe offset distance about the axis of travel between the scale axis and measurement axis and makes the indicated length either shorter or longer than the actual length.
In the past, interferometric methods and apparatus have been employed in which a coherent collimated light beam, as from a laser, has been split into two parts, one of which is given a frequency shift and the other of which is reflected from the surface under investigation, as shown in U.S. Pat. No. 4,005,936 and the phase of the modulated light carrier signal is transmitted to and reflected back from a distant reflector and compared with a reference modulated light carried signal to provide a direct read-out of distance to the reflector as shown in U.S. Pat. No. 3,619,058. A beam director for an optical ranging system also has been employed for interferometrically measuring distance changes between two retroreflectors in which the beam of radiation is introduced between the two reflectors by a steering beam splitter pellicle positioned obliquely therebetween as shown in U.S. Pat. No. 4,436,417.
In such prior art methods and apparatus various compensating means are employed to compensate for Abbe's error which results from defects of the components in a geometric measurement system.